The present invention relates to a magnification varying device for electrophotographic copying machines of the mirror scan type wherein a projection lens for converging a bundle of rays from the surface of an original is moved along the optical axis, and a deflection mirror for reflecting the bundle of rays through the projection lens toward a photoconductive drum is moved for the correction of the optical path length to give the desired altered magnification.
U.S. Pat. No. 4,571,064 (patented on Feb. 18, 1986) discloses a known device of this type.
FIG. 1 shows the basic construction of copying machines having such a magnification varying device. During copying operation, a light source a and a first movable mirror b move leftward in the drawing at a speed v/n (wherein v is the circumferential speed of a photoconductive drum c, and n is the magnification of copy to be obtained) to scan an original on a document table d. With this movement, a second movable mirror e and a third movable mirror f similarly move leftward at a speed of v/2n. In accordance with a variation in magnification, on the other hand, a projection lens g is shifted along its optical axis, and a deflection mirror h is also shifted with the lens g along the optical axis for the correction of the length of optical path, whereby the projection system is made ready to give the desired magnification.
At a magnification of 1.0.times. (equal magnification) or a reduced magnification, the light source a scans the entire copying range of the document table d, and the third movable mirror f advances with this movement to a position indicated at ms. When giving an enlarged magnification, the light source a does not scan the entire copying range of the table d, so that the most advanced position of the third movable mirror f which moves with the light source is before the position ms. At the largest magnification, for example, the advanced position is l0 as shown in FIG. 1.
The projection lens g is positioned at l1 for the largest magnification, at m1 for a magnification of 1.0.times. or at s1 for the smallest reduced magnification.
The deflection mirror h is positioned at l2 for the most enlarged magnification, at m2 for a magnification of 1.0.times. or at s2 for the most reduced magnification.
FIG. 2 shows a mechanism for shifting the projection lens g and shifting the mirror h therewith. A stepping motor i, when rotated, moves a wire j to shift the projection lens g along its optical axis, while driving an eccentric cam k to shift the mirror h which has a cam follower p in pressing contact with the cam k. Indicated at q in FIG. 2 is a tension spring for pressing the cam follower p into contact with the eccentric cam k.
With this conventional arrangement, the eccentric cam k has a cam curve of great inclination as shown in FIG. 3 and also a large radius when the range of varying magnification is, for example, as great as 0.5.times. to 2.0.times.. Now, suppose the transmission torque of the eccentric cam shaft is T, and the component of the transmitted force N in the direction of the optical axis is Nx when the cam follower p is in contact with the cam k at the position where the inclination of the cam curve is greatest (e.g. at the position of 0.5.times.). The component Nx is then given by Nx=T/R.times.cos .theta. wherein R is the radius of the cam, and .theta. is the inclination of the cam curve. Thus, the component Nx is small as shown in FIG. 3, and the force for propelling the mirror h is small, with the resulting likelihood that the stepping motor i will not operate properly owing to an excessive torque.
Furthermore, the component Ny perpendicular to the optical axis increases in corresponding relation to the decrease of the component Nx, exerting an objectionable force on the mirror h to make the mirror h difficult to move smoothly, subjecting the stepping motor i to a still increased load and distorting or vibrating the mirror h. The improper operating condition of the motor and these objections impair the quality of copies to be obtained.
As seen in FIG. 4, the projection optical system is also provided with light blocking means for regulating the light from the scanning means so that only the light substantially passing through the projection lens g reaches the mirror h, preventing the incidence of the other light. The blocking means comprises a wall r provided around the path of travel of the projection lens g for blocking the objectionable light, and light blocking members t and u provided at the front and rear ends of the projection lens 9 for blocking light at the clearance between the lens g and the blocking wall r.
Such light blocking means is effective when the magnification is to be varied over the range of from about 0.64.times. to about 1.42.times.. However, if the magnification varying range is as wide as from 0.5.times. to 2.0.times., the projection lens g needs to be moved an increased amount through an optical path of increased width. This entails the necessity of increasing the width W of the opening in the light blocking wall r or of decreasing the axial length V of the wall r. The increase in the width W diminishes the space for accommodating the mechanism for driving the lens g and the mirror h and is not advantageous, further necessitating larger elastic blocking members t and u to result in an increased drive load. Accordingly, the axial length V of the light blocking wall r is reduced as seen in FIG. 4.
When the blocking wall r is thus formed, the projection lens g is brought to a position s1 shown in FIG. 4 at the most reduced magnification, creating a large clearance A between the lens g and the wall r. Consequently, objectionable light traveling from the light toward the left in the drawing leaks from the clearance A and impinges ont he mirror h.
To close the clearance A, it appears useful to enlarge the elastic blocking member u at the rear end of the lens g and cause the member u to extend toward the third movable mirror f. Nevertheless, at a magnification of 1.0.times., the most advanced position ms of the third movable mirror f is most proximate to the position ml of the lens g (see FIG. 1), permitting the enlarged blocking member u to interfere with the mirror f. The enlarged blocking member u is therefore not usable.